This invention relates to procatalyst compositions, processes for making such compositions, and methods for using such compositions to make polymers. More particularly, the present invention relates to novel Ziegler-Natta procatalyst compositions which in combination with a cocatalyst form catalyst compositions for use in polymerization of olefins.
The properties of polymers substantially depend upon the properties of the catalysts used in their preparation. In particular, the choice of the shape, size, size distribution, and other morphological properties of supported catalysts is important to ensure operability and commercial success. This is particularly important in gas phase and slurry polymerizations. A successful catalyst composition should be based on a procatalyst particle having good mechanical properties including resistance to wear, abrasion and shattering during the polymerization process, thereby imparting good bulk density and uniformity to the resulting polymer product. Equally important are procatalyst compositions that produce such polymer products in high catalyst efficiency.
Spray-drying is a well known technique for preparing solid Ziegler-Natta polymerization procatalysts. In spray-drying, liquid droplets containing dissolved and/or suspended materials are ejected into a chamber under drying conditions to remove solvent or diluent leaving behind a solid residue. The resulting particle size and shape is related to the characteristics of the droplets formed in the spraying process. Structural reorganization of the particle can be influenced by changes in volume and size of the droplets. Depending on conditions of the spray drying process, either large, small, or aggregated particles can be obtained. The conditions may also produce particles that are compositionally uniform or contain voids or pores. The use of inert fillers in forming spray-dried particles can help control shape and composition of the resulting particles.
Numerous spray-dried olefin polymerization procatalysts containing magnesium and titanium and production processes for making and utilizing them have been reported. Examples include U.S. Pat. No. 6,187,866; U.S. Pat. No. 5,567,665; U.S. Pat. No. 5,290,745; U.S. Pat. No. 5,122,494; U.S. Pat. No. 4,990,479; U.S. Pat. No. 4,508,842; U.S. Pat. No. 4,482,687; and U.S. Pat. No. 4,302,565. Generally, such compositions have been produced in the form of substantially spheroidal shaped solid procatalyst particles having average particle diameters from 1 to 100 μm, depending on the intended end use. Porosity and cohesive strength of the particles can be adjusted by the use of fillers, such as silica, and binders, such as polymeric additives. Generally, solid rather than hollow particles are desired due to greater structural integrity of the resulting particles. Disadvantageously however, solid particles tend to have lower productivities or efficiencies due to the fact that interior regions of the procatalyst particle are not able to effectively come into contact with the cocatalyst or monomer or to otherwise participate in the polymerization process as readily as surface regions of the particle.
Despite the advance in the art obtained by the foregoing disclosures, there still remains a need to produce Ziegler-Natta procatalysts having improved performance properties. Procatalyst compositions having increased resistance to shattering and generation of polymer fines are highly desired. Generation of polymer fines is undesirable due to buildup in the polymerization equipment, thereby causing problems with bed level control and entrainment in the cycle gas leading to equipment failure, impaired operability, and reduced efficiency. High levels of fines can also cause problems in downstream handling of the polymer once it exits the polymerization system. Fines can cause poor flow in purge bins, plug filters in bins, and present safety problems. The above problems make elimination or reduction of polymer fines important to commercial operation, especially of a gas-phase polymerization process.
In a multiple series reactor system, where the composition of the polymers produced in the separate reactors is widely variable, the presence of polymer fines is particularly harmful to continuous and smooth operation. This is due to the extreme importance of precise bed level control, in as much as the product properties of the final polymer are strongly influenced by the relative amount of polymer produced in each reactor. If the bed weights are not precisely known, it is extremely difficult to properly control the final product properties.
With respect to the preparation of polyethylene and other ethylene/α-olefin copolymers, it is preferred to produce polymer in the separate reactors with both large molecular weight differences and relatively large differences in incorporated comonomer. To produce final polymers with the best physical properties, it is preferred to have one of the reactors produce a polymer with high molecular weight and incorporating a majority of any comonomer present. In the second reactor, a low molecular weight portion of the polymer is formed which may also have comonomer incorporated, but normally in an amount less than that incorporated in the high molecular weight portion. When the high molecular weight component is produced first, polymer fines can become a significant problem, especially when the flow index (I21, ASTM D-1238, condition 190/2.16) of the resulting polymer is in the range from 0.1 to 2.0 g/10 min, and the incorporated comonomer content is less than 5 weight percent, especially less than 4.5 wt weight percent.
Depending on the order of production of the different polymers in the multiple reactor system (that is production of high molecular weight polymer first and lower molecular weight polymer second or vice versa), the fines will tend to have significantly different polymer properties than the bulk of the polymer granules. This is believed to be due to the fact that the fines also tend to be the youngest particles in the reactor and hence they do not achieve conformation to the final product properties before transiting to the second reactor in series.
This in turn leads to further problems in compounding the polymer into pellets for end-use. In particular, the fines are normally of significantly different molecular weight or branching composition compared to the remainder or bulk polymer. Although the particles of both the bulk material and the fines will melt at roughly the same temperature, mixing is hampered unless the products have a similar isoviscous temperature (that is the temperature at which the melt viscosity of the two products is essentially the same). These polymer fines, which tend to be of significantly different molecular weight and isoviscous temperature than the remainder of the polymer, are not readily homogeneously mixed with the bulk phase, but rather form segregated regions in the resulting polymer pellet and can lead to gels or other defects in blown films or other extruded articles made therefrom.
Thus, generation of polymer fines is a problem, especially for gas phase olefin polymerization processes and, in particular, for staged or series reactor systems in which precise control of polymer composition is only achieved by precise control of the relative amount of polymer produced in the multiple reactors.
Accordingly, it is desirable to minimize polymer fines in an olefin polymerization process. One factor in reducing such polymer fines is by eliminating or reducing those procatalyst particles that are susceptible to the production of polymer fines due to fractioning or abrasion. To that end, one object of the invention is to provide an improved catalyst with greater mechanical strength that results in reduced polymer fines while, at the same time, possessing good polymerization response and efficiency.